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 state the law of constant proportion?give one example

Asked by Shivani Ganesh (student) , on 29/1/14

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Louis Proust stated : "A pure chemical compound, regardless of its source , is always made up of the same elements combined together in a fixed proportion by weight."

# Example : Water is composed of hydrogen and oxygen combined together in the ratio 1:8 by weight.


atomic mass of hydrogen = 2

atomic mass of oxygen = 16

when they combine, ratio = 2/16 = 1/8 = 1:8

Posted by Hunney Jain (student) on 9/1/12

The law of constant proportions, also called Proust 's law, states that the elements in a compound are all present in a fixed proportion by weight, regardless of how the compound is prepared.


Posted by Hunney Jain (student) on 9/1/12


Posted by Rajat Dhawan (parent) on 9/1/12


Law of Constant Proportion


Another French chemist, Joseph Proust, stated this law as 'A chemical compound always contains same elements combined together in the same proportion by mass '.

For example, pure water obtained from different sources such as river, well, spring, sea, etc., always contains hydrogen and oxygen together in the ratio of 1: 8 by mass. Similarly, carbon dioxide CO can be obtained by different methods such as,
  • Burning of carbon
  • Heating of lime stone
  • By action of dilute HCl on marble pieces

But the different samples of CO always contain carbon and oxygen in the ratio of 3: 8.

Posted by Param Sukhadia (student) on 8/10/12


 Law of Constant Proportion

As we know, compounds are composed of two or more elements. The proportion in which elements are present in a compound remains the same, irrespective of its method of preparation.

For example, pure water obtained from any source (well, river, lake, or sea) and from any country (India, Russia, America, etc.) will always contain two hydrogen atoms and one oxygen atom. Hydrogen and oxygen combine together in the ratio of 1:8 by mass respectively to form water, and the ratio by the number of atoms for water will always be H : O = 2 : 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen.Similarly, in ammonia, nitrogen makes up 14/17 of the mass of ammonia, while hydrogen makes up the remaining 3/17 of the mass. Irrespective of the source from which ammonia is obtained, it will always contain nitrogen and hydrogen in the ratio of 14: 3 by mass. Thus, 17 g of ammonia contains 14 g of nitrogen and 3 g of hydrogen, and 34 g of ammonia contains 28 g of nitrogen and 6 g of hydrogen. This led to the law of constant proportion.

Thus, according to the law of constant proportion, a chemical substance always contains the same elements in a fixed proportion by mass, irrespective of its source.

The law of constant proportion is also known as the law of definite proportion. This law was introduced by Proust, which stated that ‘in a compound, the elements are always present in definite proportions by mass’.

Posted by Param Sukhadia (student) on 8/10/12


Law that states that mass is neither created or destroyed in ordinary chemical and physical changes.

Posted by Guramrit Singh (student) on 31/10/12

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